/*
 * Copyright 2013 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */

package io.netty.channel;

import java.util.Map;
import java.util.WeakHashMap;

/**
 * Skelton implementation of a {@link ChannelHandler}.
 */
public abstract class ChannelHandlerAdapter implements ChannelHandler {

    /**
     * Cache the result of {@link Sharable} annotation detection to workaround a condition. We use a
     * {@link ThreadLocal} and {@link WeakHashMap} to eliminate the volatile write/reads. Using different
     * {@link WeakHashMap} instances per {@link Thread} is good enough for us and the number of
     * {@link Thread}s are quite limited anyway.
     *
     * See <a href="See https://github.com/netty/netty/issues/2289">#2289</a>.
     */
    private static final ThreadLocal<Map<Class<?>, Boolean>> SHARABLE_CACHE =
            new ThreadLocal<Map<Class<?>, Boolean>>() {
                @Override
                protected Map<Class<?>, Boolean> initialValue() {
                    // Start with small capacity to keep memory overhead as low as possible.
                    return new WeakHashMap<Class<?>, Boolean>(4);
                }
            };

    // Not using volatile because it's used only for a sanity check.
    boolean added;

    /**
     * Return {@code true} if the implementation is {@link Sharable} and so can be added
     * to different {@link ChannelPipeline}s.
     */
    public boolean isSharable() {
        Class<?> clazz = getClass();
        Map<Class<?>, Boolean> cache = SHARABLE_CACHE.get();
        Boolean sharable = cache.get(clazz);
        if (sharable == null) {
            sharable = clazz.isAnnotationPresent(Sharable.class);
            cache.put(clazz, sharable);
        }
        return sharable;
    }

    /**
     * Do nothing by default, sub-classes may override this method.
     */
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        // NOOP
    }

    /**
     * Do nothing by default, sub-classes may override this method.
     */
    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        // NOOP
    }

    /**
     * Calls {@link ChannelHandlerContext#fireExceptionCaught(Throwable)} to forward
     * to the next {@link ChannelHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        ctx.fireExceptionCaught(cause);
    }
}